Electrophysiological and behavioral data have revealed that snakes of the families Crotalidae and Boidae are able to detect distant spatial and temporal fluctuations of environmental heat distribution. This ability, involving a specialized facial pit, trigeminal system, and optic tectum allows these snakes to respond to the heat emission of their homoiothermic prey (and predators) in a fashion similar to an infrared eye. The principal aim of this proposal is to determine if the change in the function of the fifth cranial nerve and the optic tectum of infrared sensitive snakes has resulted in simple hypertrophy of the trigeminal system and optic tectum in response to their expanded function; or an evolution of an entirely new set of neural pathways; or utilization of other neural systems whose functional organization is, or was more similar to the new function of the trigeminal system in these snakes, i.e., the visual system. In order to determine which of these has occurred, projections of the trigeminal system and the optic tectum will be traced in infrared sensitive snakes as well as non-infrared sensitive snakes utilizing silver degeneration, horseradish peroxidase and autoradiographic techniques. Furthermore, an analysis will be made of the cytoarchitecture of those neural structures found to be involved in the infrared system. These experimental data will provide important insights into this unique sensory system plus providing information on trigeminal projections of reptiles and the organization of the optic tectum of snakes.